In recent years, attention is paid to microwave annealing as a method of activating the circuits in semiconductor devices. Microwave annealing can achieve activation at lower temperatures (350 to 650° C.) than optical annealing. It can therefore control the diffusion of impurities, thereby to lower the leakage current. Further, a microwave reaches the deepest layer of a multi-layered circuit because its wavelength (several centimeters to hundreds of centimeters) is longer than the wavelength of light. It may therefore achieve uniform activation of any multi-layered circuit with a high probability. Hence, the microwave annealing process is anticipated to be of use in manufacturing three-dimensional semiconductor devices.
However, microwave annealing rarely achieves activation if the microwave has a frequency of 1 GHz or less. In this case, it takes longer to accomplish sufficient activation, as proven by experiments. On the other hand, if the frequency of the electromagnetic wave is raised to a light-frequency region, for example 300 THz or more (equivalent to a wavelength of 1 μm or less), low-temperature activation will not occur. Unless the temperature is set to 1000° C. or more, activation will not be accomplished in fact. In this case, the impurities will inevitably diffuse, possibly imposing adverse influence.
In order to solve this problem, the frequency of the microwave may be tuned to an optimal value. It is, however, very difficult to perform optimal tuning owing to variation in the type of semiconductor substrate, and amount of non-single-crystal silicon, as well as other factors. As a result, it is extremely difficult to tune the microwave frequency for the respective semiconductor substrates.